By Kay Quinn, St. Louis Post-Dispatch, June 26, 2006

Whether you''re an elite athlete or a child attending outdoor day camp, not drinking enough fluid in the summertime heat can affect everything from how you feel and think, to how you perform on the field.

Beth Herndon, 20, is a senior at Washington University in St. Louis. She knows she''s been dehydrated. As a member of the university''s cross-country and track teams, she sometimes runs 60 to 70 miles a week.

"I''ve definitely had times where you can tell that you haven''t had enough to drink," said Herndon, "and I''ve had problems breathing or just feel really run down."

That decline in performance due to inadequate fluid intake can also affect your ability to think and make good judgments.

It can take 24 hours to rehydrate your body after sweating for an extended period of time in high temperatures.

Dehydration can also have a cumulative effect. If you''re active over several consecutive days and you don''t get enough water or replenishing fluids, you can accumulate a fluid deficit.

"Most of the time, people will start to notice they''re getting a little more thirsty than usual," explained Dr. Mark Halstead, an orthopedic surgeon at Barnes-Jewish Hospital in St. Louis. "But that''s the sign to us that maybe they''re getting a little too far behind, and you should have been drinking a little beforehand."

Here are some guidelines for staying hydrated in the heat.

• Drink 12 to 24 ounces of water in the hour before exertion in the heat.
• During exertion, try to drink 4 ounces of fluid every 15 minutes.
• Drink an additional 10 ounces of fluid after exertion.

Halstead says either water or sport drinks work best. Stay away from high-sugar drinks, and those with caffeine or alcohol. Keep in mind that you can lose as much as 3 percent of your body weight when you are active for an hour in 80-degree weather.